"Fionn" means "blond", "fair", "white", or "bright". The hero's childhood name was Deimne (/ˈdeɪni/; Irish pronunciation: [dʲeβ̃nʲi]),[2] literally "sureness" or "certainty", also a name that means a young male deer; several legends tell how he gained the name Fionn when his hair turned prematurely white. The name Fionn is related to the Welsh name Gwyn, as in the mythological figure Gwyn ap Nudd, and to the continental Celtic Vindos, a form of the god Belenus.

The 19th-century Irish revolutionary organisation known as the Fenian Brotherhood took its name from these legends. The Scottish form of his name, Fingal/ˈfɪŋɡəl/, comes from a retelling of the legends in epic form by the 18th-century poet James Macpherson.

Muirne was already pregnant; her father rejected her and ordered his people to burn her, but Conn would not allow it and put her under the protection of Fiacal mac Conchinn, whose wife, Bodhmall the druid, was Cumhall's sister. In Fiacal's house Muirne gave birth to a son, whom she called Deimne.

Muirne left the boy in the care of Bodhmall and a fighting woman, Liath Luachra, and they brought him up in secret in the forest of Sliabh Bladma, teaching him the arts of war and hunting. As he grew older he entered the service – incognito – of a number of local kings, but each one, when he recognised Fionn as Cumhal's son, he told him to leave, fearing they would be unable to protect him from his enemies.

The young Fionn met the leprechaun-like druid and poet Finn Eces, or Finnegas, near the river Boyne and studied under him. Finnegas had spent seven years trying to catch the Salmon of Knowledge, which lived in a pool on the Boyne and became all-knowing through its diets of hazelnuts from a holy tree: whoever ate the salmon would gain all the knowledge in the world. Eventually Finn Eces caught it, and told the boy to cook it for him. While cooking it Deimne burned his thumb, and instinctively put his thumb in his mouth. This imbued him with the salmon's wisdom, and when Finn Eces saw that he had gained wisdom, he gave young Fionn the whole salmon to eat. Fionn then knew how to gain revenge against Goll, and in subsequent stories was able to call on the knowledge of the salmon by putting his thumb to the tooth that had first tasted the salmon.[3] The story of Fionn and the salmon of knowledge and the Welsh tale of Gwion Bach are similar.

Every year for 23 years at Samhain, a fire-breathing man of the Sidhe, Aillen, would lull the men of Tara to sleep with his music before burning the palace to the ground, and the Fianna, led by Goll mac Morna, were powerless to prevent it. Fionn arrived at Tara, armed with his father's crane-skin bag of magical weapons. He kept himself awake by touching the point of his magically red-hot spear to his forehead. The pain kept Fionn awake, allowing him to pursue and kill Aillen with the same spear. After that his heritage was recognised and he was given command of the Fianna: Goll willingly stepped aside, and became a loyal follower of Fionn, although in some stories their alliance is uneasy. Fionn demanded compensation for his father's death from Tadg, threatening war or single combat against him if he refused. Tadg offered him his home, the hill of Allen, as compensation, which Fionn accepted.

Fionn met his most famous wife, Sadhbh, when he was out hunting. She had been turned into a deer by a druid, Fear Doirich, whom she had refused to marry. Fionn's hounds, Bran and Sceólang, born of a human enchanted into the form of a hound, recognised her as human, and Fionn brought her home. She transformed back into a woman the moment she set foot on Fionn's land, as this was the one place she could regain her true form. She and Fionn married and she was soon pregnant. When Fionn was away defending his country, Fear Doirich (literally meaning Dark Man) returned and turned her back into a deer, whereupon she vanished. Fionn spent years searching for her, but to no avail. Bran and Sceólang, again hunting, found her son, Oisín, in the form of a fawn; he transformed into a child, and went on to be one of the greatest of the Fianna.

In The Pursuit of Diarmuid and Gráinne the High King Cormac mac Airt promises the ageing Fionn his daughter Gráinne, but at the wedding feast Gráinne falls for one of the Fianna, Diarmuid Ua Duibhne, noted for his beauty. She forces him to run away with her and Fionn pursues them. The lovers are helped by the Fianna, and by Diarmuid's foster-father, the god Aengus. Eventually Fionn makes his peace with the couple. Years later, however, Fionn invites Diarmuid on a boar hunt, and Diarmuid is gored. Water drunk from Fionn's hands has the power of healing, but each time Fionn gathers water he lets it run through his fingers before he gets back to Diarmuid. His grandson Oscar shames Fionn, but when he finally returns with water it is too late; Diarmuid has died.

According to the most popular account of Fionn's death, he is not dead at all, rather, he sleeps in a cave, surrounded by the Fianna. One day they will awake and defend Ireland in the hour of her greatest need. In one account, it is said they will arise when the Dord Fiann, the hunting horn of the Fianna, is sounded three times, and they will be as strong and as well as they ever were.[4] Another tradition states that he is buried in the crypt of Lund Cathedral in Sweden.

Many geographical features in Ireland are attributed to Fionn. Legend has it he built the Giant's Causeway as stepping-stones to Scotland, so as not to get his feet wet; he also once scooped up part of Ireland to fling it at a rival, but it missed and landed in the Irish Sea — the clump became the Isle of Man and the pebble became Rockall, the void became Lough Neagh. Fingal's Cave in Scotland is also named after him, and shares the feature of hexagonal basalt columns with the nearby Giant's Causeway in Northern Ireland.

In both Irish and Manx popular folklore,[5] Fionn mac Cumhail (known as "Finn McCool" or "Finn MacCooill" respectively) is portrayed as a magical, benevolent giant. The most famous story attached to this version of Fionn tells of how one day, while making a pathway in the sea towards Scotland – The Giant's Causeway – Fionn is told that the giant Benandonner (or, in the Manx version, a buggane) is coming to fight him. Knowing he cannot withstand Benandonner due to his size, Fionn asks his wife Oona to help him. She dresses her husband as a baby, and he hides in a cradle; then she makes a batch of griddle-cakes, hiding griddle-irons in some. When Benandonner arrives, Oona tells him Fionn is out but will be back shortly. As Benandonner waits, he tries to intimidate Oona with his immense power, breaking rocks with his little finger. Oona then offers Benandonner a griddle-cake, but when he bites into the iron he chips his teeth. Oona scolds him for being weak (saying her husband eats such cakes easily), and feeds one without an iron to the 'baby', who eats it without trouble.

In the Irish version, Benandonner is so awed by the power of the baby's teeth and the size of the baby that, at Oona's prompting, he puts his fingers in Fionn's mouth to feel how sharp his teeth are. Fionn bites Benandonner's little finger, and scared of the prospect of meeting his father considering the baby's size, Benandonner runs back towards Scotland across the Causeway smashing the causeway so Fionn couldn't follow him.

The Manx Gaelic version contains a further tale of how Fionn and the buggane battle at Kirk Christ Rushen. Fionn's feet carve out the channels between the Calf of Man and Kitterland and between Kitterland and the Isle of Man, while the buggane's feet make an opening for the port at Port Erin. The buggane injures Fionn, who flees over the sea (where the buggane cannot follow), but the buggane tears out one of his own teeth and strikes Fionn as he runs away. The tooth falls into the sea, becoming the Chicken Rock, and Fionn curses the tooth, explaining why it is a hazard to sailors.

In Newfoundland, and some parts of Nova Scotia, "Fingal's Rising" is spoken of in a distinct nationalistic sense. Made popular in songs and bars alike, to speak of "Fingle," as his name is pronounced in English versus "Fion MaCool" in Newfoundland Irish, is sometimes used as a stand-in for Newfoundland or its culture.

In 1761 James Macpherson announced the discovery of an epic written by Ossian (Oisín) in the Scottish Gaelic language on the subject of "Fingal". The personal name Fingal is derived from the Gaelic Fionnghall,[6] and it has been suggested that Macpherson rendered Fionn's name as Fingal through a misapprehension of the various forms of Fionn.[7] In December 1761 Macpherson published Fingal, an Ancient Epic Poem in Six Books, together with Several Other Poems composed by Ossian, the Son of Fingal, translated from the Gaelic Language. His cycle of poems was wildly popular and had widespread influence on such writers as Goethe and the young Walter Scott, but there was controversy from the outset about Macpherson's claims to have translated the works from ancient sources. The authenticity of the poems is now generally doubted, though they may have been based on fragments of Gaelic legend, and to some extent the controversy has overshadowed their considerable literary merit and influence on Romanticism.

Glencoe: The Story of the Massacre by John Prebble (Secker & Warburg, 1966), has an account of a legendary battle between Fionn mac Cumhaill, who supposedly lived for a time in Glencoe (in Scotland), and a Viking host in forty longships which sailed up the narrows by Ballachulish into Loch Leven. The Norsemen were defeated by the Feinn of the valley of Glencoe, and their chief Earragan was slain by Goll MacMorna.

Contemporary Scottish poet Marie Marshall has written a semi-serious ballad in parody of 19th-century neo-medievalism How Finn McCool became Lord of Tara. It tells the story of Fionn saving the house of Tara from the spell of Allan-of-the-Harp, an elf-king with a hatred of human prosperity. A sample passage runs:

In the 1999 Irish dance show Dancing on Dangerous Ground, conceived and choreographed by former Riverdance leads, Jean Butler and Colin Dunne, Tony Kemp portrayed Fionn in a modernised version of The Pursuit of Diarmuid and Gráinne. In this, Diarmuid, played by Colin Dunne, dies at the hands of the Fianna after he and Gráinne, played by Jean Butler, run away together into the forests of Ireland, immediately after Fionn and Gráinne's wedding. When she sees Diarmuid's body, Gráinne dies of a broken heart.

In 2010, Washington DC's Dizzie Miss Lizzie's Roadside Revue debuted their rock musical Finn McCool at the Capitol Fringe Festival. The show retells the legend of Fionn mac Cumhaill through punk-inspired rock, and was performed at the Woolly Mammoth Theater in March 2011.[9]